Soldiers of the 31st Pennsylvania Infantry during the American Civil War. Location unknown, USA, circa 1861.

Soldiers of the 31st Pennsylvania Infantry during the American Civil War. Location unknown, USA, circa 1861.

Photo: Fotosearch, Getty Images

Soldiers of the 31st Pennsylvania Infantry during the American... Photo-4870828.65685 - San Antonio Express-News

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Tent life of the 31st Penn. Inf. (later, 82d Penn. Inf.) at Queen's farm, vicinity of Fort Slocum, Washington, DC, 1861. The husband and wife stand with children and a baby. The wife does the wash and is surrounded by kitchen utensils.

Tent life of the 31st Penn. Inf. (later, 82d Penn. Inf.) at Queen's farm, vicinity of Fort Slocum, Washington, DC, 1861. The husband and wife stand with children and a baby. The wife does the wash and is

circa 1860s: A woman visitor to Union Army headquarters at Miner's Hill, Virginia. Although women were not always welcome, this one may be the wife of an officer or an agent of the US Sanitary Commission.

circa 1860s: A woman visitor to Union Army headquarters at Miner's Hill, Virginia. Although women were not always welcome, this one may be the wife of an officer or an agent of the US Sanitary Commission.

Allan Pinkerton (1819 - 1884), Scottish-born founder of the Pinkerton National Detective Agency, with some of his agents, circa 1862. Pinkerton served as head of the Union Intelligence Service during the American Civil War and foiled a plot to assassinate Abraham Lincoln.

Allan Pinkerton (1819 - 1884), Scottish-born founder of the Pinkerton National Detective Agency, with some of his agents, circa 1862. Pinkerton served as head of the Union Intelligence Service during the

Railroad officials pointing to holes left by Confederate gunshot in engine & tender of Federal military train, during Civil War. (Photo by Time Life Pictures/National Archives/Time Life Pictures/Getty Images)

Railroad officials pointing to holes left by Confederate gunshot in engine & tender of Federal military train, during Civil War. (Photo by Time Life Pictures/National Archives/Time Life Pictures/Getty Images)

George A. Custer (1839 - 1876), then a Captain, on horseback, left, and facing General Alfred Pleasanton (1824 - 1897) , Falmouth, VA, 1863. Pleasanton, a cavalry general during the American Civil War, promoted

George A. Custer (1839 - 1876), then a Captain, on horseback,... Photo-4870782.65685 - San Antonio Express-News

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A soldier holding up the torn flag of the 8th Pennsylvania Reserve Colour Guard during the American Civil War.

A soldier holding up the torn flag of the 8th Pennsylvania Reserve Colour Guard during the American Civil War.

Photo: MPI, Getty Images

A soldier holding up the torn flag of the 8th Pennsylvania Reserve... Photo-4870789.65685 - San Antonio Express-News

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circa 1863: Steam locomotive 'Fire Fly', used by Union troops, on a narrow field bridge of the Orange and Alexandria US Military Railroad, during the American Civil War. (Photo by Mathew Brady/Henry Guttmann/Getty Images)

Officer assembling an 8-in-long torpedo, which was designed by Union Brig. Gen. Herman Haupt for purpose of destroying bridges. (Photo by Time Life Pictures/National Archives/Time Life Pictures/Getty Images)

Officer assembling an 8-in-long torpedo, which was designed by Union Brig. Gen. Herman Haupt for purpose of destroying bridges. (Photo by Time Life Pictures/National Archives/Time Life Pictures/Getty Images)

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Officer assembling an 8-in-long torpedo, which was designed by... Photo-4870822.65685 - San Antonio Express-News

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Captain Harry Page, Quartermaster of the Headquarters of the Army of the Potomac, reclines on the field, likely awaiting the arrival of his wagon train, in order personally to superintend the pitching of the tents and parking of the wagons, May 1864. He has doubtless ridden forward to the position selected for camp, to examine its capabilities. His horse stands at rest, and an orderly keeps an eye about the vicinity. (Photo by Timothy H. O'Sullivan/George Eastman House/Getty Images)

Captain Harry Page, Quartermaster of the Headquarters of the Army of the Potomac, reclines on the field, likely awaiting the arrival of his wagon train, in order personally to superintend the pitching of the

Captain Harry Page, Quartermaster of the Headquarters of the Army... Photo-4870833.65685 - San Antonio Express-News

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General Ulysses S Grant, commander of the Union Army during the American Civil War, at Cold Harbor Virginia, circa 1864. This image is from the files of the United States National Archives.

General Ulysses S Grant, commander of the Union Army during the American Civil War, at Cold Harbor Virginia, circa 1864. This image is from the files of the United States National Archives.

Photo: Popperfoto, Getty Images

General Ulysses S Grant, commander of the Union Army during the... Photo-4870784.65685 - San Antonio Express-News

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circa 1865: Full-length image of two American soldiers touching their boxing gloves together as they pose before their match in front of a barracks during the Civil War era. Two soldiers crouch on either side of the boxers.

circa 1865: Full-length image of two American soldiers touching their boxing gloves together as they pose before their match in front of a barracks during the Civil War era. Two soldiers crouch on either side

Copy of a photograph showing a group portrait of Union Army generals, from left, Wesley Merritt, Philip Sheridan, George Crook, James William Forsyth, and George Armstrong Custer, gathered around a table examining a document, Washington, DC, 1865. (Photo by Alexander Gardner/Interim Archives/Getty Images)

Copy of a photograph showing a group portrait of Union Army generals, from left, Wesley Merritt, Philip Sheridan, George Crook, James William Forsyth, and George Armstrong Custer, gathered around a table